1. Field of the Invention
This invention relates to an apparatus and method for reconstructing two-dimensional pictures of object slices within a human or other body by means of passing penetrating radiation, typically X-radiation or gamma radiation, through the plane of the body from a plurality of angles. The field is commonly known as computerized tomography or transverse axial tomography.
2. Description of the Prior Art
In present applications of X-ray computerized tomography, X-ray source energies which are useful for medical diagnosis are inherently panchromatic, i.e. the incident X-ray spectrum is not all focused at one energy, but rather is in a spectral shape representing a whole host of different energies. FIG. 1. This has unfortunate ramifications for the quality of the output pictures thus obtained because all of the reconstruction methods in current use, regardless of source-detector geometry, proceed on the assumption that the incident X-ray beam is monoenergetic. Thus it has been recognized that it would be desirable to remove from the output displays those artifacts caused by the panchromatic nature of the incident beam. An early approach to the solution of the problem revolved around the use of a waterbag. The problem is discussed in McCullough et al., "An Evaluation of the Quantitative and Radiation Features of a Scanning X-Ray Transverse Axial Tomograph: The EMI Scanner," Radiology 111:709-716, June, 1974.
A previous sketchy approach to this problem can be found in a paper entitled "Utilization of Simple Energy Spectrum Measurements in X-ray Computerized Tomography", by Robert E. Alvarez and Albert Macovski, appearing at MB1-1 in the Technical Digest for the Conference, "Image Processing for 2-D and 3 -D Reconstruction from Projections: Theory and Practice in Medicine and the Physical Sciences", August 4-7, 1975, Stanford University, Palo Alto, California. This publication suffers from several infirmities. First, there is no mention of any technique for treatment of the physical phenomenon known as Rayleigh scattering. Second, there is no mention of how detectors may be utilized to obtain different spectral information. Third, there is no analysis of the errors which are generated using the technique described. Fourth, in description of the test which was performed, FIG. 1 shows output sketches of a photoelectric phenomenon and a Compton scattering phenomenon, with no relation of these drawings to the atomic number of the molecules in the object under study or the electron density of same. Fifth, the calibration and generation of output pictures was achieved using computer simulated data, not actual data actually measured by a tomographic scanner.